The T/t-complex of the mouse has a role in the specification of sets of cell surface antigens that appear important in cell interactions and recognition during early development [1]. Having already demonstrated that abnormal cell surface antigens associated with specific embryonic lethal t-haplotypes have different oligosaccharide determinants [2[, I will now try to define these antigens structurally. My recent experiments have shown that lectin columns can be used to partially purify relevant antigens from testicular cell lysates, and that using such partially purified material from t12 testicular cell lysates to immunize rabbits yields antiserum with specific cytotoxic activity for t12 testicular cells. Immunoprecipitation experiments with this antiserum show that the major component precipitated is a glycoprotein of 87,000 daltons, which is obtained from both t12 and normal testicular cells although in substantially greater quantity from t12 cells. Thus, the opportunity exists to isolate and compare normal and mutant cell surface antigens. The immunoprecipitated proteins will be analyzed and compared in detail with respect to (a) their carbohydrate:protein ratio, (b) carbohydrate structure, and (c) protein backbone structure. Taking into account the nature of the T/t-complex and its effects on embryonic differentiation, the isolation, identification and characterization of T/t-complex associated antigens should reveal cell surface components essential for early development. These studies should shed light on the molecular mechanisms that govern cell-cell interaction and recognition during embryonic development.